1. Field of the Invention:
This invention relates to a meter having a pointer actuated by a magnetic field, and to a driving system for such a meter.
2. Description of the Related Art:
the velocity or engine speed of a vehicle is usually indicated by a pointer of a speedmeter or a tachometer. A cross-coil meter is widely used for this purpose.
FIG. 2 of the accompanying drawings shows the operation principle of such a cross-coil meter. The cross-coil meter includes a magnet M and two exciting coils Ls and Lc. The coils Ls, Lc generate magnetic fields having intensities according to currents supplied by a driving system (not shown). These two coils are arranged to be perpendicular to each other. A combined magnetic field composed of the magnetic fields of these coils Ls, Lc is controlled by regulating the currents which are applied to these coils. The magnet M is located so as to be actuated by the combined magnetic field. When it is actuated, the magnet M generates torque, angularly moving according to the torque. A non-illustrated pointer is connected to the magnet M to deflect in response to the angular movement of the magnet M.
The pointer deflects according to the currents applied to the coils Ls, Lc. Therefore, when the currents to the coils Ls, Lc change in response to the measured vehicle or engine speed, the pointer deflects to indicate the varying vehicle or engine speed. FIG. 3 of the accompanying drawings shows a speedometer for a vehicle. In FIG. 3, a pointer I is connected to the magnet M of FIG. 2. An angle .theta. corresponds to the angle by which the magnet M is moved due to the combined magnetic field generated by the coils Ls, Lc.
A pulse output type sensor is usually used for detecting the vehicle or engine speed. Some speed sensors generate pulse signals having different frequencies according to detected speeds. When a cross-coil meter is set in motion by an output from such a sensor, it is necessary to use a counter for counting the frequency of pulses, a modulator for performing the pulse width modulation (PWM) according to the counted results of the counter, a driver for supplying PWM-modulated currents to the coils Ls, Lc, and so forth. To deflect the pointer I by the angle .theta. according to the value detected by the sensor, a ROM or the like should be used to correspond the counted frequencies with the deflection angle .theta..
Usually inputs from the sensor are variable to a certain extent. To operate the cross-coil meter, such variable inputs should preferably be stabilized to prevent unfavorable deflection of the pointer I. For this purpose, the counted frequency of pulses are conventionally averaged in a specified period of time, so that the deflection angle .theta. of the pointer is controlled by a mean value. A frequency Fn at a certain time point is determined by the following formula. EQU Fn=(F.sub.in +F.sub.n-1 +F.sub.n-2 +. . . +F.sub.n-a)/(a+1)
where a is an integer, F.sub.in is an input frequency, F.sub.n-1 is a frequency which serves as a base for determining the deflection angle .theta. at the time point n-1, F.sub.n-2 is another frequency serving as another base for determining the deflection angle .theta., and so forth.
When this equation is employed, the calculation results (F.sub.n-1 to F.sub.n-a) should be obtained until the time point a, which means a slow response. To improve the response, a formula for obtaining a weight average has been studied and in use now. EQU F.sub.n =.alpha.F.sub.n-1 +(1-.alpha.)F.sub.in
where .alpha. is a constant (0&lt;.alpha.&lt;1).
However, even when F.sub.n is obtained by using the second formula, the foregoing calculation has to be repeated each time a new frequency is counted by the counter, resulting in too quick a response in some cases, or too slow a response in other cases. For example, when a cross-coil meter is operated by a sensor for outputting high frequency pulses, the number of calculations per unit time is increased to make the pointer I fluctuate extensively. Otherwise, the number of calculations per unit time becomes too few to reduce the pointer's responsiveness.